The present invention relates to a diagnostic method for a production line and, in particular, to an improvement in a method of locating a malfunctioning section in a production line controlled by a sequencer.
As is known, a production line, such as an assembly line for automobiles, may be equipped with a sequence control section having a built-in computer. By means of this sequence control section, sequence control is carried out over the operations to be sequentially performed by various items of equipment of the production line. In effecting such sequence control, the built-in computer of the sequence control section is loaded with control programs, in accordance with which the sequence control section successively carries out the different stages of operational control over the various items of equipment installed in the production line.
In many cases, such sequence control, performed over the operations of various items of equipment installed in a production line, is accompanied by failure diagnosis, which is carried out to monitor the control condition, detecting any failure in the equipment. Such diagnosis associated with sequence control may take a variety of forms. For example, Japanese Patent Laid-Open No. 60-238906 proposes a diagnostic method, according to which the modes of operation of the components of a sequence control circuit section when the associated equipment is operating in the normal manner are preset as reference modes of operation, and the modes of operation of the components of the sequence control section when the equipment is actually in operation are compared one by one with the reference modes of operation mentioned above, whereby any failure in the equipment is detected on the basis of any difference between these two categories of operation mode thus compared with each other.
However, the equipment of a production line is composed of a variety of components, which means the actual modes of operation of the equipment vary to a large degree, whereas the number of reference modes of operation for the equipment, with which the actual modes of operation for the equipment are to be compared, is limited. Thus, it may happen that the actual modes of operation of the components of the sequence control circuit section do not agree with the preset reference modes of operation, with the result that the equipment is erroneously judged to be out of order although it is really operating in the normal manner.
Further, when a failure has been detected in the equipment, it is necessary to locate the operation element thereof which has caused the failure. In locating an operation element which has caused a failure, it is necessary to check the mode of operation judged to be discordant with the corresponding reference mode of operation sequentially with a large number of operating elements of the equipment related to that mode of operation. Thus, in many cases, it is difficult to quickly locate the operation element which has caused the failure.
As described above, the method disclosed in Japanese Patent Laid-Open No. 60-238906 is effective for a failure which causes the equipment to stop its operation completely. However, for slight failures which are not so serious as to cause the equipment to stop completely, for example, failures attributable to secular change, which cause the equipment to take somewhat longer to operate than in the normal state, the above method is substantially incompetent since it is impossible to store all the patterns of such slight failures in memory as reference modes of operation.
The so-called "trap method" is a diagnostic method proposed as a means of detecting such slight failures. In the trap method, a supervisory ladder program is prepared, in accordance with which a timer is started upon output to an output device in a sequence-control ladder program, and it is checked whether or not a confirmation device for confirming the operation of the output device operates before the timer has run out.
The problem with the trap method described above is that a ladder program for trapping must be prepared for each of the individual ladder program elements, resulting in a very poor efficiency.
Furthermore, in an actual production line, the actuators may vary in terms, for example, of weight and operating speed. As a result, it may happen that an L/S switch gets into a logically impossible condition, as frequently experienced by the present inventors. That is, the actuator stops in a condition corresponding to neither "forward" nor "return" state, or gets into a condition corresponding to both "forward" and "return" state. This is partly due to a bounding of the actuator. Suppose, for example, the actuator has returned to the "return" state, with the LS for "return" detection operating to cause a "return" signal to be detected, upon which the sequence proceeds further. However, it often happens that the actuator bounds immediately after the sequence has thus proceeded further, the actuator slightly returning in the "forward" direction. Because of this slight returning, no "return" state detection signal is generated by the LS for detecting "return" movement of the actuator, with the result that the actuator gets into a condition which corresponds to neither "forward" nor "return" state.
Even when the switch has gotten into such a halfway condition, the sequence will proceed, until an operation step is reached where an operation is started upon checking the condition of that switch. However, the point at which the sequence thus stops is in all probability very far away from that operation step in which the failure has actually occurred, with the result that it is difficult to perform appropriate failure diagnosis.